1. Field of the Invention
This invention generally relates to inertial sensors, and more particularly, to an inertial sensor such as an acceleration sensor and a gyro having an excellent impact resistance.
2. Description of the Related Art
The inertial sensor such as the acceleration sensor and the gyro has been downsized, more sophisticated, and reduced in price in these years, with advancements of microfabrication technology employing MEMS (Micro-Electro-Mechanical System) techniques. With the above-mentioned background, the inertial sensor as a MEMS device is expected to be used for a car navigation system, automotive air-bag control, avoidance of jiggling a camera or camcorder, mobile telephone, robot posture control, gesture input recognition for a game, and detection of HDD rotation and impact applied to HDD. The inertial sensor is expected to be employed in every device for detecting movements.
An unexpected big impact is sometimes applied to the device that detects the movements. Therefore, there is a possibility that the big impact is applied to the inertial sensor mounted on the above-mentioned device. For instance, with respect to the inertial sensor used for an automobile such as a vehicle air bag, if a car crushes or overturns, the big impact, never happening during the normal driving, will be possibly applied. If a mobile device is mistakenly dropped, the big impact can be easily imagined. If a user roughly handles a game console for a hobby use, an extremely big impact will be applied to the game console. The above-mentioned accidental and unexpected impact is considered 3000 G or 5000 G, and a high impact resistance is demanded by the inertial sensor mounted on the above-mentioned device. A common inertial sensor has a fundamental structure including the weight, which is the movable portion, hung by the beams. In the case where the big impact is accidentally applied to the device, there is a known problem in that the beam is significantly distorted or damaged and the device no longer serves as a sensor. Japanese Patent Application Publication No. 2000-187041 (hereinafter referred to as Document 1) describes to solve the above-mentioned problem.
The capacitive acceleration sensor disclosed in Document 1 includes a protector provided on an insulating protective cover to sandwich a movable portion. If a big impact is applied and the movable portion hits the insulating protective cover, the movable portion is configured not to be damaged by the impact. According to Document 1, this protector protects the acceleration sensor from damage of the movable portion or malfunction of the acceleration measurement, if a device on which the sensor is mounted is dropped.
The acceleration sensor disclosed in Document 1, however, intends to solve the problem of the acceleration sensor having a configuration in which “the mass body 4a is supported in parallel and can be displaced by the interfaces between the glass substrates 6 and 7 and the silicon substrate 1 via the beam 4c” (refer to the paragraph 0005 of Document 1). In addition, the acceleration sensor has a configuration in which “the beam supporting the mass body 4Aa has a narrow width in proportion to the thickness thereof, and the mass body 4Aa is configured to have a difficulty of being displaced toward the glass substrates 6A and 7A” (refer to the paragraph 0026 of Document 1). The movable space of the movable portion is limited to a two-dimensional space substantially parallel to the interfaces of the glass substrates 6 and 7 and the silicon substrate 1.
Further, “the concave portion 6Aa having a depth of approximately 15 μm is formed on the surface facing the convex portion 4Ab of the mass body 4Aa in the glass substrate 6A. The aluminum layer 8 having a thickness of approximately 5 μm is coated as a protector to protect against the impact by vapor evaporation” (refer to the paragraph 0027 of Document 1). Thus obtained acceleration sensor does not have a possibility that the movable portion is damaged and becomes unable to measure the acceleration if the device on which the sensor is mounted is dropped. That is, the protector provided in the acceleration sensor disclosed in Document 1 is arranged on the glass substrate, on which the movable portion is not displaced in the normal operation of the acceleration sensor. There is a problem in that the invention disclosed in document 1 cannot be applied to an inertial sensor that serves primary functions when the movable portion moves in three dimensions in the normal operation.